1. Field of the Invention
The present invention relates to an image processing method and apparatus for correcting an image signal, which is obtained from a solid-state detector.
2. Description of the Related Art
In a medical radiological imaging device or the like, a radiological solid-state detector having a photoconductive layer such as a selenium plate, which is sensitive to radiation such as X-rays, is used as an image detector so that a radiation dose received by a subject is reduced and the diagnostic performance of the device is improved, for example. The radiological solid-state detector is irradiated with radiation, and an electric charge corresponding to the irradiated radiation dose is accumulated as a latent image charge in a charge accumulating portion within the radiological solid-state detector. Accordingly, radiological image information is recorded in the charge accumulating portion as an electrostatic latent image. The radiological image information is also read out from the solid-state detector by scanning the radiological solid-state detector, in which the radiological image information is recorded, by read-out light, which is either beam-shaped or line-shaped. Accordingly, the radiological image information is read out from the solid-state detector. The method as described above is already disclosed (U.S. Pat. Nos. 5,331,179 and 4,857,723).
In the method as described above, the image information obtained from the solid-state detector represents the amount of an electric charge, which is accumulated in the solid-state detector. Since the image information is an analog image signal, the image information needs to be digitized (A/D conversion) to perform image processing or the like on the obtained image. As methods for digitizing analog image signals, there are a method using linear quantization and a method using non-linear quantization. The non-linear quantization is effective in reducing a S/N ratio in a low signal level region. Therefore, when the analog image signal is converted into a digital image signal, the non-linear quantization by using logarithmic transformation (logarithmic compression) is performed.
As methods for performing the non-linear quantization as described above, there are a method for performing logarithmic transformation after performing A/D conversion on an analog image signal, and a method for performing A/D conversion after performing logarithmic transformation on an analog image signal by using a logarithmic amplifier. In the former method, there is a problem that a conversion error generated during the A/D conversion is amplified when the logarithmic transformation is performed. Since the error is amplified, the resolution deteriorates, and granularity increases in a low density region, which is represented by the image signal. Therefore, a higher bit A/D converter or, if the bit number is the same, an expensive A/D converter, of which conversion error is small, is required to reduce the quantization error. Hence, there is a problem that the cost becomes high.
In the latter method, since the A/D conversion is performed after performing the logarithmic transformation, the conversion error is not amplified. However, since the logarithmic amplifier is required, there is a problem that the cost becomes high.